This invention relates to garnet materials and, more particularly, to gadolinium scandium gallium garnet.
Garnet materials have a number of diverse technological applications. They are used, for example, as the substrate for magnetic bubble domain devices and as the host for the active light-emitting species of solid state lasers.
Gadolinium scandium gallium garnet (GSGG), which was first prepared by C. D. Brandle et al. (Journal of Crystal Growth, Vol. 20, p. 1, (1973)) using the Czochralski technique, recently has been used as a host material for Cr-doped tunable lasers (P. F. Moulton, Laser Focus, p. 83, May 1983) and for Nd:Cr-doped high power solid state lasers (E. V. Zharikov et al., Soviet Journal of Quantum Electronics, Vol. 14, p. 1056 (1984)). However, uniform crystals are important for laser applications. Achieving uniformity requires an accurate knowledge of the congruently melting composition; that is, the composition at which the growing crystal has the same composition as the melt from which it is grown.
The garnet structure is a complex oxide network that contains three dodecahedral, two octahedral, and three tetrahedral sites per formula unit. The natural site distribution is gadolinium (ionic radius r.sub.i =1.053 Angstrom) on the large dodecahedral site, scandium (r.sub.i =0.745 Angstrom) on the medium sized octahedral site, and gallium (r.sub.i =0.47 Angstrom) on the small tetrahedral site. However, it is known from other garnets that there can be small off-site distributions of these ions, namely: (1) Gallium is readily incorporated on the octahedral site in a variety of rare earth gallium garnets; (2) Gadolinium is incorporated octahedrally in gadolinium gallium garnet (GGG), C. D. Brandle et al., Journal of Crystal Growth, Vol. 26, p. 169 (1974); and (3) Scandium is incorporated dodecahedrally in gadolinium scandium aluminum garnet, C. D. Brandle et al. (1973), supra. Any or all of these mechanisms may affect the congruent composition of GSGG. The total distribution coefficients of all three components must be equal to unity to grow a crystal of uniform composition.
Previous researchers used lattice parameter measurements to determine the compositional uniformity of GSGG. Brandle et al. (1973), supra, compared their lattice parameter data to their calculated values and concluded that there must be octahedral gadolinium in these crystals. K. Chow et al. (Journal of Crystal Growth, Vol. 23, p. 58, (1974)) used lattice parameter data from sintered (ceramic) samples to conclude that the congruent melting composition was Gd.sub.3.0 Sc.sub.1.6 Ga.sub.3.4 O.sub.12. Such determinations based on one variable are not sufficient to solve a three component system, especially with the added complication of off-site distributions of more than one ion. Consequently, we have determined that the Chow et al. congruent melting composition is inaccurate and that GSGG crystals grown in reliance on it suffer from considerable lack of uniformity.